In a boom-type crane, the boom is pivotally mounted on a chassis or other support structure for swinging movement about a horizontal axis, the load being applied to the free end of the boom through a cable or the like. The system for actuating the boom may include a double-acting piston-and-cylinder arrangement engaging the boom and adapted to swing the latter under the control of a manual element, e.g. a control lever, through a range of angular positions at which the boom includes more or less steep angles with the horizontal.
It will be apparent that a given load applies to the boom a variable load moment depending upon the boom position, the load moment being a function of the downward force times at horizontal distance between the pivot and the point of attack of the downward force. In other words, the horizontal component of the moment at any instant defines the load moment for a given load. As the boom swings upwardly for a given load, the load moment decreases and, conversely, as the boom swings downwardly, the load moment increases.
For each angular position of the boom, the crane (the boom supporting structures and chassis) has a maximum permissible load moment which cannot or should not be exceeded if the danger of breakdown or tipping is to be avoided.
It is not uncommon, therefore, to provide, as part of the crane control system, means for monitoring the loading (i.e. the load moment) applied to a crane boom.
The conventional drive for a crane boom may comprise, as noted, a hydraulic motor such as the double-acting cylinder mentioned previously, which serves as the output element of a servocontrol system whose input element is the manually operated control lever mentioned previously. This element provides a set point value to the control system and the controller causes displacement of the hydraulic motor until the position of the boom corresponding to the set point input is reached.
The load moment monitor is usually independent of this boom-actuating system and generally includes a further system in the form of a switch which is operated when the load moment becomes excessive to cut out undesired further displacement of the boom.
The difficulty with this system, in which independent lockout of the operation of the boom when the instantaneous load moment reaches the maximum permissible load moment, is that it is not responsive to the operator. As a consequence, if the operator should shift the lever into high speed lowering of the boom or another rapid displacement which might tend to overrun the speed with which the monitoring device can act, the boom will nevertheless be shifted past its maximum permissible load moment.
In other words, in order for the conventional device to function effectively, the operator cannot rapidly displace the boom when the monitoring device indicates that the instantaneous load moment is close to the maximum permissible load moment.
When care is not taken, the crane will tip if the maximum permissible load moment is exceeded because the monitoring or cutoff device could not react with sufficient speed.
To avoid this problem it has been proposed to provide an acoustical or optical warning signal which is triggered when the instantaneous load moment approaches the maximum permissible load moment and which further can provide an alert when the maximum permissible load moment reaches high values, i.e. where the danger of tipping or the like is more pronounced. Frequently the warning is not heeded.